To avoid this problem, various proposals have been made previously to provide air-in-line detectors so that the presence of any air bubbles in a liquid line leading to a patient can be detected and an alarm raised. Air-in-line detectors have to detect the presence of air in both opaque liquids such as blood and transparent or translucent liquids, and consequently, such detectors are relatively complicated. It is also very desirable for the detectors to be fail-safe so that an alarm condition is raised when any single component within the device fails to ensure that the patient is safeguarded to the maximum extent. One of the results of such existing systems is that repeated false alarms occur, and this is dangerous because repeated false alarms can result in a correct alarm signal being ignored.
One example of an air-in-line detector for detecting the presence of air in a translucent tube containing liquid is described in GB-A-1550060. This specification describes a fluid detector comprising a channel for receiving the tube and five radiation emitter-detector pairs; three of which are arranged to detect the presence of an opaque liquid with the emitter and detector of each pair being located on opposite sides of the channel so that radiation emitted by the radiation emitters impinges directly on their associated detectors and two of which are arranged to detect the presence of translucent liquids with the emitter and detector of each pair being located on opposite sides of the channel but not directly aligned so that radiation emitted by the radiation emitters only impinges on its associated detector after refraction by the tube filled with a translucent liquid.
This existing device also includes a logic circuit which analyzes the outputs of the detectors and provides an alarm when air is detected in the transparent tube. The logic circuit includes a steering gate coupled to the top detector and depending upon whether the top detector receives a light or dark response this preconditions the remainder of the logic circuit to look for air in an opaque liquid or look for air in a transparent liquid. The other detectors are used as two separate pairs each pair of which detects air in the line by looking for an air bubble of sufficient size to be present in the path of both detectors of that pair.
The detector described thus automatically takes account of whether the liquid in the tube is opaque or translucent and can take account of some single component failures. However, for example, if the lowest radiation emitter ceases to emit radiation, or the radiation path between this and its associated detector is blocked by debris or, for example, the detector fails and is permanently open circuit, the logic circuit fails to detect any problem. Further, if such a faulty system is used with an opaque liquid, the top radiation emitter-detector pair preconditions the logic circuit to look for coincidence of signals from one or other of the two lower radiation detector radiation emitter pairs to indicate the presence of an air bubble large enough to bridge the detectors in one of the pairs.
What should happen now is that an alarm should be raised, but the faults described above would prevent the bottom detector even giving a signal, and thus, the required coincidence never occurs and consequently unlimited amounts of air could be infused into a patient without an alarm signal being given. This is not the only single component failure which could be life threatening in the arrangement shown and described in this patent specification.
This conventional air-in-line detector also, erroneously gives an alarm indication if a transparent liquid replaces an opaque liquid in the tube and, vice versa since the logic circuit interprets the interface between the two liquids as the presence of an air bubble. A further limitation of this existing device is that it can only detect the presence of an air bubble of a particular size, namely of a size sufficient to bridge two successive radiation-detector emitter pairs and this is determined by the physical construction of the device. Clearly, it would be possible to have a number of substantially sized, but spaced, air bubbles in the tube and, under these circumstances such a string of bubbles would probably not initiate an alarm and yet their cumulative volume would be sufficient to be life threatening.